This invention pertains to implantable cardioversion devices (ICDs) which sense a dangerous cardiac arrhythmia and, in response, provide therapy to the patient""s heart to revert it to a normal sinus rhythm. More particularly, the invention pertains to implantable medical devices and methods, such as an ICD including a sensor for sensing intrinsic cardiac activity and a cardioverter/defibrillator adapted to provide different types of antitachycardia therapy dependent upon the condition of the heart as indicated by the sensor. The ICD is further provided with an automated threshold adjustment means for setting the threshold(s) delimiting the therapies.
As used herein, the term xe2x80x9cabnormal arrhythmiaxe2x80x9d refers to any abnormal heart rhythm that may be dangerous to the patient and specifically includes fibrillation, tachycardias, supraventricular tachycardias (SVT), ventricular tachycardias (VT), ventricular fibrillation and flutter (VF), and bradycardia. As further used herein, the term xe2x80x9ctherapyxe2x80x9d refers to any means used by the ICD to restore normal heart rhythm such as defibrillation, cardioversion, antitachycardia pacing (ATP), antibradycardia therapy and drug infusion. The disclosed invention has application to ICDs which treat tachyarrhythmias (abnormally high heart rates).
It has been common practice to monitor the heart rate, or more commonly the ventricular rate, of a patient and classify the cardiac condition of the patient based on this heart rate. (Other criteria, in addition to the ventricular rate, may also be used for this classification, but these criteria are omitted for the sake of clarity). For example, tachyarrhythmia may be defined as any rate in a range above a designated threshold VT1. This range is then divided into ventricular tachycardia and ventricular fibrillation (and flutter) zones. The ventricular tachycardia zone may be further divided into slow ventricular tachycardia and fast ventricular tachycardia zones.
Once it is determined that a patient suffers from one of these cardiac conditions, the ICD is programmed to provide a corresponding therapy. Typically, ventricular tachycardia is treated with a therapy consisting of low-energy pacing pulses designed to capture the ventricle. This therapy is referred to as AntiTachycardia Pacing therapy (ATP). Ventricular fibrillation, on the other hand, is treated more aggressively with high energy shocks. The ICD is programmed with parameters for various types of therapies and the rates defining the therapy zones corresponding to the respective therapies.
Over the years, the number of programmable parameters has been increasing steadily. A modern ICD has up to 200 or more programmable parameters. A major challenge for both the ICD manufacturer and the clinician is to select proper values for these parameters. While the manufacturer may provide nominal or default values for the parameters, these nominal values may not be proper for all patients and it is up to the clinician to change them using statistical information and his personal experience. However, changing and adapting the parameters have proven to be difficult and it has been found that most clinicians leave the majority of the parameters at their nominal values.
Some of the parameters, however, including the thresholds defining the antitachycardia therapies described above, should be changed periodically to conform to the changing condition of the patient.
It is, hence, desirable to provide an ICD capable of selecting or adjusting some of its parameters automatically so that the clinician does not have to set them on implantation or adjust them each time the condition of the heart changes. More importantly, it would be advantageous to provide an ICD which can adjust some of its parameters adaptively, quickly and efficiently setting them at their optimal levels, and resetting or re-adjusting them automatically as the condition of the patient changes.
The present invention addresses the problem of automatically and dynamically adjusting the thresholds which define the various tachyarrhythmia therapy zones.
In its broadest sense, the present invention pertains to an ICD which includes a timing and control circuit adapted to classify the condition of a patient as being one of certain preselected types of cardiac conditions based essentially, but not necessarily exclusively, upon the heart rate. The ICD further includes a circuit for selecting and setting thresholds which define the therapy zones corresponding to cardiac conditions. These thresholds are adaptively changed to conform to the individual characteristics and requirements of the patient. In this manner, the ICD can be programmed with initial thresholds, selected either by the manufacturer or the clinician, which are based on statistical information from other patients, etc. These thresholds are then automatically adjusted by the ICD over time. Thus, the clinician does not need to change the default settings or reprogram the ICD during a subsequent visit if the condition of the patient changes.
More particularly, the present invention contemplates an ICD comprising a sense amplifier that generates a sense signal indicative of intrinsic events in the ventricle; a control and timing circuit which generates control signals responsive to the sense signal to define a tachyarrhythmia therapy; a pulse generator that receives the control signals and generates corresponding output signals delivered to the patient""s heart; and a threshold setting circuit for setting the thresholds which define the various zones characteristic of specific cardiac conditions associated with tachyarrhythmia. Importantly, a success rate circuit is also provided which monitors whether a particular therapy, defined by the control signals, is successful. This information is used to determine whether a particular threshold should be changed and, if so, whether the particular threshold should be raised or lowered. Preferably, the ICD is also provided with a memory for storing various programming and operational parameters, as well as a set of rules for setting each threshold.